1. Field of the Invention
The present invention relates to a crankcase of an internal combustion engine, and particularly relates to fastening structure of a part supporting a crankshaft in the crankcase.
2. Background Art
An engine structure, including a crankcase employing a fastening structure for fastening a crankcase and a crank cap in a circumference of a crankshaft is known. The known fastening structure includes providing an opening formed at the end of, and communicating with, a bolt hole which receives the fastening member therein. An engine structure of this type is described, for example, in published Japanese patent document JP-A No. H10-122042.
In a fastened part of a crankshaft wall, which rotatably supports the crankshaft, a load applied to the crankshaft has a great effect upon the fastened part. In the engine structure disclosed in published Japanese patent document JP-A No. H10-122042, the end of a bolt remains inside the crankcase, and stress may build up and become concentrated in the crankcase at a location corresponding to a circumference of the bolt end. Therefore, the rigidity of the crankcase is required to be increased, the weight of the crankcase necessarily increases, and it is difficult to reduce the weight of the crankcase, which affects vehicle weight and fuel economy.
The crankcase described in published Japanese patent document JP-A No. H10-122042 is roughly equivalent to an upper crankcase in the present specification, and the crank cap in published Japanese patent document JP-A No. H10-122042 is roughly equivalent to a lower crankcase in the present specification.